1. Field of the Invention
This invention relates to electric-wave filters, and more particularly, to improvements in the art of contiguous frequency-channel separation multiplexers/demultiplexers.
2. Description of Related Art
Electric-wave filters are often configured for the purpose of separating contiguous frequency-channels (demultiplexing) or for the purpose of combining such channels (multiplexing or multicoupling). For demultiplexing, a single wide-bandwidth composite input signal is split into multiple narrower-bandwidth component output signals, each capable of following separate signal-flow paths. Conversely, for multiplexing, multiple input signals, each occupying a different frequency channel, are efficiently combined so that they may flow together on a single output path as a wider-bandwidth composite signal. Typically a frequency filter which accomplishes demultiplexing uses reciprocal components, thereby allowing the same filter to perform multiplexing by simply reversing the direction of signal flow. Examples of such application are frequency multiplexers and demultiplexers used at each end of a single coaxial-cable communication link to enable the link to simultaneously carry many communication channels. Frequency demultiplexers are also used in channelized receivers to separate signals of different frequency so that the signals can be further processed by parallel circuity.
The prior art provides effective means for multiplexing or demultiplexing when the number of channels is small. Reference is made particularly to both parallel-connected and series-connected multiplexers described by Matthaei, Young and Jones in Chapter 16 of their book Microwave Filters, Impedance-Matching Networks, and Coupling Structures (Artech House, Inc., Dedham, MA, 1980). However, the prior art is not efficient or effective in cases where the number of channels is large because of the large number of filters that are required, and the steps that are necessary for eliminating interactions between the filters.
One example of a prior art method for implementing a multiplexer/demultiplexer with many channels is shown by the block diagram in FIG. 1 wherein a multiplexer/demultiplexer is depicted as a series cascade of multiple sections, each section including a branching filter 102 (104, 106, 108) and a series isolator 101 (103, 105,107). The branching filter 102 typically consists of multiple coupled resonators, tuned to pass signals within the frequency band of the channel being branched. The isolator 101 is any one of a variety of devices, for example, a ferrite non-reciprocal type or an active non-reciprocal type such as an amplifier (both examples being applicable to multiplexing or demultiplexing but not both simultaneously), a frequency-selective filter type, or a quadrature hybrid type in a balanced configuration. The purpose of the isolator is to reduce impedance interactions and thus prevent the detuning of each branching filter by the presence of the other branching filters. A major disadvantage of this approach is that the channels at the far end of the cascade suffer the sum of the insertion losses of each preceding section.
Another prior art demultiplexer is also formed as a series cascade. However, in this case directional filters which have constant resistance input impedances are cascaded so that additional isolation devices are not required. Nevertheless, the insertion loss of each directional filter is significant such that channels near the end of the cascade suffer high insertion loss. Additionally, it is difficult to tune the resonators within each directional filter because of the simultaneous presence of orthogonal modes.
Another example of a prior-art method for implementing a multiplexer/demultiplexer with many channels is shown by the block diagram in FIG. 2 wherein a demultiplexer is depicted as a corporate network of binary band-splitters 201, 202, 203, 204, 205. Each band-splitter is a diplexing pair of filters which are matched to the transmission lines at input and outputs by virtue of complementary impedance design or by use of separate means of impedance isolation. Each band-splitter passes an input signal to the left or to the right outputs depending on whether the signal frequency is in the lower or upper portion of the frequency band assigned to that band-splitter. Frequency bands are assigned in accordance with the position of the band-splitter within the demultiplexer. For example, the band-splitter 201 at the input level must cover the whole band of interest, while the two band-splitters 202 and 203 at the next level each cover complementary halves of the band of interest (actually each is arranged to cover slightly more than half the band so that its passband extends beyond the crossover frequency of the previous band-splitter; this staggering of passband edges avoids the high insertion loss that would otherwise result at certain channel edges if each band-splitter's bandwidth were a binary multiple of the channelwidth). This corporate type architecture has an advantage over the series cascade architecture in that the sum of the insertion losses in the corporate case is the same for each channel and is appreciably lower than the average loss in the cascade architecture for the same number of channels. Nevertheless, the insertion loss of the corporate-type architecture may be higher than desirable if each bandsplitter has even moderate insertion loss. Achieving low insertion loss in each band-splitter requires that the resonators, which comprise the band-splitter, have a very high Q-factor; this in turn requires that the resonators be of large physical size. Thus the requirement for this prior art demultiplexer to have low insertion loss is coupled with the disadvantage that its size must be large.